The topography of lipopolysaccharide (LPS) bilayers as well as the topography and elasticity of membrane vesicles (MV) of Pseudomonas aeruginosa Gram-negative bacteria have been investigated with the atomic force microscope. The LPS bilayer thickness was consistent with stereochemical estimates of the lipid A and core regions of the LPS layer. From measurements on MVs, a thickness of 7.5 nm for the outer membrane was determined. Thickness measurements on dried and hydrated MVs further showed that varying amounts of periplasmic molecular components are encapsulated by MVs. When spherical MVs in liquid attach to a surface, they distort into the shape of a lens. Force spectroscopy on such MVs showed a springlike elastic response. This suggests that the MV envelope is under tension. Analysis of the spring constants in terms of a simple model gave MV osmotic pressures on the order of atmospheres.